This chapter explores legal and theoretical aspects of naturalization. The first section addresses the ultimate goal of naturalization—what function does it serve?—by presenting three goals: contract, political test, and nation-building. Each goal may lead to a different process of naturalization and raise different ethical questions. The literature does not systematically discuss which goals are legitimate, and which ones are illegitimate. The second section seeks to present three ways to assess the ethics of naturalization—drawing on conceptual and utilitarian grounds. Among the conceptual factors to be considered are the legitimacy of the goals of naturalization according to different conceptions of citizenship (liberalism, republicanism, cosmopolitanism, and communitarianism) and of nationhood (primordial, civic, and cultural). Among the utilitarian considerations are the efficiency of the naturalization criteria in achieving a legitimate goal. The third section moves on to examine three trends in naturalization policy in Western societies—legalization, devaluation, and liberalization (followed by a restrictive turn). It points out that naturalization has been internationalized in the direction of creating a right to citizenship; that citizenship is becoming a ‘commodity’ in the global economy market whose nature is increasingly influenced by economic factors; and that the process of liberalization in access to the status of citizenship is facing a restrictive turn—cultural considerations are becoming more central in naturalization decisions. The chapter concludes by offering new directions for the study of naturalization.